1. Field of the Invention
This invention is concerned with electroluminescent displays, and more specifically to displays that are electroluminescent/liquid crystal hybrids.
2. Description of the Related Art
Certain materials are electroluminescent—that is, they emit light, and so glow, when an electric field is generated across them. The first known electroluminescent materials were inorganic particulate substances such as zinc sulphide, while more recently-found electroluminescent materials include a number of small-molecule organic emitters known as organic light emitting diodes (OLEDs) and some plastics—synthetic organic polymeric substances—known as light-emitting polymers (LEPs). Inorganic particulates, in a doped and encapsulated form, are still in use, particularly when mixed into a binder and applied to a substrate surface as a relatively thick layer; LEPs can be used both as particulate materials in a binder matrix or, with some advantages, on their own as a relatively thin continuous film.
This electroluminescent effect has been used in the construction of displays. In some types of displays an electroluminescent (EL) material—generally referred to in this context as a phosphor—is provided to form a backlight which can be seen through a mask that defines whatever characters the display is to show. In other types, the displays are arranged with shaped electrodes such that small individual areas of EL material can be selectively activated. These displays have many applications; examples are a simple digital time and date display (to be used in a watch or clock), a mobile phone display, the control panel of a household device (such as a dishwasher or washing machine), and a handheld remote controller (for a television, video or DVD player, a digibox, stereo or music centre or similar entertainment device).
International patent application No: WO 2005/0121878 describes an electroluminescent display (Hybrid Display) with a Liquid Crystal (LC) mask that is switchable in individual areas, between “on”/transparent (so that the backlight can shine therethrough) and “off/opaque (so that the backlight's light is blocked thereby). The LC mask and the EL backlight are formed as a single integral unit wherein both components (the LC mask and EL backlight) are operated using common electrodes—the EL backlight being created as an electroluminescent material layer mounted directly on the rear of or behind the layer of liquid crystal material.
It will be appreciated that it is essential that the LC material be in a physically-stabilised form rather than in the normal “liquid”, mobile, form implied by its name.
The inventor has realised that a polymer dispersed liquid crystal (PDLC) film consisting of droplets of liquid crystal, typically nematic or cholestic in nature, dispersed in a polymer matrix (binder) is suitable for the LC mask of such a hybrid display.
By including a dichroic dye in the PDLC, the unpowered state can be strongly absorbing as well as scattering, whilst the powered state combines transparency with low absorption. This type of PDLC is known as NCAP (entrapped nematic curvilinear aligned phase) and the inventor has found the NCAP type of PDLC to be particularly suitable for use as the LC material in the hybrid display. In particular, NCAP PDLCs minimise dye migration into the binder, and so do not degrade the achievable contrast.
These films can be constructed using an emulsification method, or by using one of a number of phase separation techniques. The emulsification method uses mechanical shear to disperse dyed liquid crystal organic oil into an aqueous-based medium comprising a water-soluble polymer, for example polyvinyl alcohol (PVA), which is to form the polymer matrix.
The formation of the display can be achieved by a number of methods. WO 2005/0121878 discloses a method wherein each of the various layers is screen-printed into place (apart from the ITO front electrode, which is usually sputtered onto the substrate), through masks that define the shape, size and position of each layer of the display, using suitable pastes that are subsequently dried, set or cured, as appropriate, prior to the next layer or collection of layers being applied.
The inventor has found that using an NCAP PDLC material with PVA as a binder is unsatisfactory because, even though the PVA is an excellent binder, providing very stable emulsions, it absorbs water readily from the atmosphere, which in turn degrades the electro-optic performance of the PDLC through water dependent leakage currents. Accordingly, displays manufactured using PVA as a binder in the LC layer have poor environmental stability, which can result in defects, such as spotting of the display.
UK patent application No: 0625114.4 proposes a solution to this problem wherein the PDLC layer comprises UV curable polyurethane. A display having such a PDLC layer may have increased environmental stability and improved electro-optical behaviour.
It has been found that hybrid displays comprising a PDLC layer fade when exposed to elevated temperatures, such as 50 or 60° C. This fading has been observed as an increase in the reflectivity of the display, particular in non-illuminated areas of the display. This increase in reflectivity decreases the contrast between the illuminated and non-illuminated areas of the display and hence reduces legibility of the display. It has also been observed that the print quality of the phosphor layer printed onto the PDLC is lower than a phosphor layer printed onto an ITO layer.
Investigation has showed that the fading is not caused by degradation of the dye used to colour the liquid crystal but by migration of the dye and liquid crystal out of the PDLC layer, through the EL layer, to an insulator layer used as a capping layer on the rear of the display. This insulator layer becomes coloured as a result of the migration of the dye. Fourier Transform Infrared Spectroscopy (FTIR) has been used to demonstrate the presence of liquid crystal in the insulator layer of a test lamp that had been aged at 85° C. for approximately 18 h rs.